Celtic conceded three goals in a horror Europa League first half against Roma as Wilfried Nancy became the first manager of the club to lose his opening two games amid a turbulent eight days in charge.

After defeat against Hearts in the Scottish Premiership on his debut last Sunday, Nancy's side trailed early after Liam Scales headed into his own goal from a corner.

The misery was compounded by a double from boyhood Celtic fan Evan Ferguson.

That put the game well beyond the Scottish champions even before Arne Engels struck a post with a penalty in stoppage time at the end of the first period, summing up the home team's ineptitude.

The ironic cheering of the Roma fans rang round the stadium moments before a chorus of boos accompanied the Parkhead side off the pitch.

No further damage was done by the impressive Italian visitors during a second half in which Roma's Leon Bailey and Celtic's Kelechi Iheanacho both had goals ruled out.

However, defeat has dented Celtic's hopes of progressing to the knockout stage, with Nancy's side poised perilously just inside the qualification places with two games remaining.

The ease with which Roma sliced through Celtic was as worrying as the amount of times they gave the ball away or were dispossessed by the visitors' eager press.

Celtic's resistance, if you could call it that, was broken after only six minutes. A corner from the right was headed beyond his own keeper by Scales.

Ferguson's double was simplicity personified, Roma exploiting the spaces they had in the wide areas and feeding the striker with opportunities to score.

Nancy watched the carnage unfolding in front of him impassively, arms folded. No in-game changes, little apparent in the way of in-game management.

That changed at half time, with Kieran Tierney, Benjamin Nygren and Daizen Maeda replaced by Colby Donovan, Paulo Bernardo and Iheanacho.

The substitutes made a difference, with Celtic less porous at the back and more of a threat up front. Iheanacho thought he had scored after providing a cute finish to a nice move, only to be thwarted by a VAR offside ruling.

A similar thing happened to Roma, who thought they had a deserved fourth when substitute Bailey bent in from inside the area, only for him to suffer the same fate.

Still, the Serie A side cruised to their second win in Glasgow in a matter of months after their 2-0 victory in this competition at Ibrox.

Celtic's new manager will need to find some dramatic improvements, and quickly, before things unravel in front of his eyes in just his first couple of weeks in charge.

His decision to stick with the same starting XI who lost to Hearts backfired, with one of Celtic's worst first-half performances in recent memory. It would be hard to overstate the manner in which they were totally outplayed.

Three cheap goals goals conceded. Numerous other chances afforded. A really good Kasper Schmeichel save. The failure of record signing Engels to hit the target with a penalty.

Fielding only three defenders in each of his first two games has brought nothing but pain for Nancy, who seemed content to let the first period play out even when it was very clearly not going to plan.

If he's stress-testing his players, several have failed. Using wingers Yang Hyun-Jun and Sebastian Tounekti as the last line of defence in wide areas to accommodate his 3-4-3 system is surely unsustainable.

Roma were streets ahead in every department, despite coach Gian Piero Gasperini opting to leave Bryan Cristante, Lorenzo Pellegrini, Kostas Tsimikas, Manu Kone, Paulo Dybala and Wesley on the bench.

What the Italian giants might have done to Celtic with their best side on the pitch barely bears thinking about.

As lessons go, this was a sore one. As learning curves go, this is as steep as it gets for the new Celtic manager.

Two big games, two damaging defeats, both at a disbelieving Parkhead, where frustrated fans are looking for reasons to vent further at the club's heirarchy. St Mirren will approach Sunday's League Cup final with optimism.

Celtic's players who look like a shadow of those who Martin O'Neill steered to seven wins out of eight and Nancy will have learned plenty about them.

The changes he makes for the Hampden showpiece - assuming he makes any at all - could determine whether he's holding silverware at the end of his first full week in charge or holding his head in exasperation at a start out of his worst nightmares.

Celtic manager Wilfried Nancy: "The reality is we were not able to cope with the intensity. First half was difficult, we were not able to come out of the pressure.

"It wasn't easy for my players. I don't like to talk about luck. The first goal didn't help us.

"Second half was better. I cannot tell you [the players] didn't try, they tried. I'm not concerned, I really liked the reaction. They deserve at least to score one goal and the dynamic could change.

"The result isn't what we want but I've seen good things. I would like the results to change today. This is a bit difficult because my players deserve a bit more. The belief of my players is really strong.

"I respect the fans, they are really important for us. We play for them. I know where we are going. I can see a lot of good things. I believe in what we do."

Celtic's next destination is the national stadium on Sunday and a showdown with St Mirren in the League Cup (15:30 GMT).

They don't return to Europa League duty until the new year, when they visit Bologna on January 22 and welcome Utrecht a week later.